But people who marry without living together first are just as likely to separate as those who cohabit then marry.

A study in the Office for National Statistics' (ONS) Population Trends bulletin says people who cohabit for the first time are up to nine times more likely to split up as married people.

And those living together for the second time are up to six times as likely to separate as those in a second marriage.

But the study suggests people take living together seriously. Three quarters of British people have only ever lived with one person. Less than 1% of adults have lived with five or more partners.

A third of people who married after living together said they took the plunge to strengthen their relationship.

One in five tied the knot for the sake of their children.

The number of people who live together has risen dramatically in recent years, but most go on to get married or break up.

By the late 1980s, only about 40% of first partnerships were marriages that did not involve living together first.

But a further 40% were marriages that did involve pre-marital cohabitation.

Only 20% were cohabiting couples.

Europe

Another ONS study, by Kathleen Kiernan of the London School of Economics, shows that the UK is one of the few countries in Western Europe where the number of single people is rising.

Most ethnic minorities live in Greater London

But like the UK, western and northern European countries are more likely to cohabit than the more conservative southern Europeans.

In most countries, cohabitation is for a short period. Either the couple choose to get married or split up.

In all countries, people whose parents have divorced are more likely to live together than those whose parents stayed married.

Lucy Sellick, a counsellor at relationship charity Relate, said she saw many cohabitees and married people, as well as people with commitment problems.

She believes a major reason why cohabitees have relationship problems is that one partner wants to get married and the other does not, perhaps because they are afraid of the commitment.

This could be because of "emotional backage" which often relates to their childhood experiences.

"There is a feeling of insecurity with cohabitation," she said. "One or both of the people involved may feel the relationship is not permanent."

However, she added that cohabitation was a serious commitment, often involving a mortgage.

"The fact that I see a lot of cohabitees shows how seriously they take the relationship.

"They are equally as upset as married people when it is over," she said.

Minorities

Population Trends also contains a study of ethnic minority populations in the UK.

It shows that the number had risen to 3.6m by 1997 or 6.4% of the population.

Almost half originated from the Indian subcontinent and just under half were born in the UK.

Most had a younger age profile than the white population and nearly 50% lived in Greater London.

Eighty-five per cent of black Africans and 60% of black Caribbeans in the UK lived in the London area.

However, less than 20% of people of Pakistani origin lived in the capital.

The ONS bulletin also predicts the number of people in England and Wales living to the age of 100 will rise from 6,000 in 1996 to 39,000 in 2036 and 95,000 by 2066.

However, few people are likely to live to be over 123 by the end of the next century.

The number of centenarians has roughly been doubling every 10 years since the 1940s.

The main reason is falling death rates for people aged 80 to 99. Other causes are an increase in the birth rate in the 19th century and improved survival from birth to the age of 80.

Other new statistics for England and Wales show:

• The number of births fell by 1% between 1997 and 1998, but fertility patterns by age are around the same
• Mothers are giving birth nearly three years later than in the early 1970s. The average age of mothers at birth is 28.9 years
• The number of births outside marriage continues to rise
• Mortality levels are highest in north-east England and north-west England and lowest in south-west England
• The London Borough of Newham has the highest fertility rate and Cambridge and Rutland the lowest.